Some key dates about railroad history:
1825 is the first intercity railway (Stockton and Darlington).
1830 is the first all-steam intercity railway (Liverpool and Manchester).
1863 is the first metro line (the Metropolitan line in London, mostly under the New Road).
The main AHC is to give upper and lower bounds for key changes in these dates, including the routes and technologies chosen. For example, I'd argue that as soon as the New Road was laid down in the late 18c, it was destined to be the first alignment for urban rail in London, just because the area was wealthy enough that mainline railroads could not be built closer to the City or the West End. I even think Liverpool and Manchester have to be the first two cities connected by steam rail once it became clear that England would lead the Industrial Revolution (so, early 18c and the decline of the Dutch Republic) - they're at the correct distance and elevation change for early rail to be feasible and much more useful than canals.
1825 is the first intercity railway (Stockton and Darlington).
1830 is the first all-steam intercity railway (Liverpool and Manchester).
1863 is the first metro line (the Metropolitan line in London, mostly under the New Road).
The main AHC is to give upper and lower bounds for key changes in these dates, including the routes and technologies chosen. For example, I'd argue that as soon as the New Road was laid down in the late 18c, it was destined to be the first alignment for urban rail in London, just because the area was wealthy enough that mainline railroads could not be built closer to the City or the West End. I even think Liverpool and Manchester have to be the first two cities connected by steam rail once it became clear that England would lead the Industrial Revolution (so, early 18c and the decline of the Dutch Republic) - they're at the correct distance and elevation change for early rail to be feasible and much more useful than canals.